The present invention relates to a hydraulic brake system using a hydraulic booster which boosts leg-power exerted on a brake pedal to a predetermined value, in which pressurized fluid introduced into a power chamber of the hydraulic booster is introduced into brake cylinders in order to actuate brakes, and more particularly to a hydraulic brake system in which master cylinder pressure developed in a master cylinder is supplied to brake cylinders through a switching valve when fluid pressure drops, thereby ensuring positive operation of brakes.
Sometimes employed in a vehicle is a hydraulic brake system which uses a hydraulic booster, which boosts leg-power exerted on a brake pedal to a predetermined value by pressurized fluid. In such a brake system, the pressurized fluid introduced into a power chamber of the hydraulic booster is introduced into brake cylinders in order to actuate brakes. Such a brake system can provide sufficient braking force with small leg-power on the brake pedal, thereby ensuring positive operation of the brakes and reducing the driver's labor.
In such a hydraulic brake system, it is desired to ensure positive operation of the brakes even when the fluid pressure drops. As one of conventional hydraulic brake systems which can ensure the positive operation of the brakes even when the fluid pressure drops, proposed in Japanese Unexamined Patent Publication No. 64-47659 is a hydraulic brake system which operates brakes by supplying fluid pressure developed in a hydraulic booster by a switching valve to wheel cylinders during normal operation, and ensure positive operation of the brakes by supplying master cylinder pressure developed in a master cylinder to wheel cylinders when fluid pressure drops.
In the hydraulic brake system disclosed in this publication, the operation of the switching valve is controlled by the fluid pressure introduced into a power chamber of the hydraulic booster during the braking operation. To be described in detail, the system has a piston disposed in the switching valve for controlling the operation of the switching valve and the piston is provided with a large-diameter portion and a small-diameter portion. When the braking operation is not performed, the piston is set in a position where the communication between the power chamber of the hydraulic booster and the wheel cylinders is interrupted and the communication between a fluid chamber of the master cylinder and the wheel cylinders is allowed, while when the braking operation is performed and the pressurized fluid is thereby introduced into the power chamber, the fluid pressure in the power chamber is applied to the large-diameter portion of the piston so as to set the piston in a position where the communication between the fluid chamber of the master cylinder and the wheel cylinders is interrupted and the communication between the power chamber of the hydraulic booster and the wheel cylinders is allowed.
However, in the conventional hydraulic brake system, the switching valve is set to interrupt the communication between the power chamber of the hydraulic booster and the wheel cylinders and allow the communication between the fluid chamber of the master cylinder and the wheel cylinders when the braking operation is not performed.
Therefore, when the braking operation is performed for normal braking under conditions of normal fluid pressure, the pressurized fluid is not supplied to the wheel cylinders until the switching valve is switched even after the fluid pressure is introduced into the power chamber of the hydraulic booster. Accordingly, the response of this system is not necessarily good. In addition, the direction, in which the fluid pressure is exerted on the large-diameter portion of the piston of the switching valve to switch the switching valve, opposes the direction, in which the fluid pressure of the pressurized fluid to be supplied to the wheel cylinders is exerted on the small-diameter portion, thereby causing loss in the fluid pressure. This is also a reason making the response insufficient.
When the fluid pressure drops, the fluid pressure in the master cylinder is supplied to the wheel cylinders through the switching valve. The master cylinder pressure is exerted on the piston in such a manner that the piston is moved in such a direction as to interrupt the communication between the fluid chamber of the master cylinder and the wheel cylinders. As the force of a spring is set to large in order to prevent the moving of the piston, the piston of the switching valve is difficult to move quickly when the hydraulic booster is actuated under conditions of normal fluid pressure, thereby further making the response worse. It is therefore not simply solved by just setting the force of the spring larger. Moreover, it is quite difficult to determine the force of the spring biasing a piston and a pressure receiving area of fluid pressure of the piston in such a manner as to securely and rapidly move the piston of the switching valve in the direction that allows the communication between the power chamber of the hydraulic booster and the wheel cylinder and interrupts the communication between the fluid chamber of the master cylinder and the wheel cylinders when the hydraulic booster is actuated under conditions of normal fluid pressure, and securely not to move the piston in the direction that allows the communication between the power chamber of the hydraulic booster and the wheel cylinder and interrupts the communication between the fluid chamber of the master cylinder and the wheel cylinders when the fluid pressure drops.
There is another problem that components of the switching valve such as a seal of the piston are inferior in durability because the piston of the switching valve moves in such a direction as to increase the volume of the fluid chamber of the master cylinder so as to increase the pedal stroke for normal braking and the piston moves every time the hydraulic booster is actuated.